Method and system for generating droplets for EUV photolithography processes

ABSTRACT

An extreme ultraviolet (EUV) photolithography system generates EUV light by irradiating droplets with a laser. The system includes a droplet generator with a nozzle and a piezoelectric structure coupled to the nozzle. The generator outputs groups of droplets. A control system applies a voltage waveform to the piezoelectric structure while the nozzle outputs the group of droplets. The waveform causes the droplets of the group to have a spread of velocities that results in the droplets coalescing into a single droplet prior to being irradiated by the laser.

BACKGROUND

There has been a continuous demand for increased computing power inelectronic devices including smart phones, tablets, desktop computers,laptop computers and many other kinds of electronic devices. Integratedcircuits provide the computing power for these electronic devices. Oneway to increase computing power in integrated circuits is to increasethe number of transistors and other integrated circuit features that canbe included for a given area of semiconductor substrate.

The features in an integrated circuit are produced, in part, with theaid of photolithography. Traditional photolithography techniques includegenerating a mask outlining the pattern of features to be formed on anintegrated circuit die. The photolithography light source irradiates theintegrated circuit die through the mask. The size of the features thatcan be produced via photolithography of the integrated circuit die islimited, in part, on the lower end, by the wavelength of light producedby the photolithography light source. Smaller wavelengths of light canproduce smaller feature sizes.

Extreme ultraviolet (EUV) light is used to produce particularly smallfeatures due to the relatively short wavelength of EUV light. Forexample, EUV light is typically produced by irradiating droplets ofselected materials with a laser beam. The energy from the laser beamcauses the droplets to enter a plasma state. In the plasma state, thedroplets emit EUV light. The EUV light travels toward a collector withan elliptical or parabolic surface. The collector reflects the EUV lightto a scanner. The scanner illuminates the target with the EUV light viaa reticle. However, if the droplets are not properly formed andirradiated, then there may be insufficient EUV light to perform an EUVprocess. Accordingly, the photolithography processes may fail and theresulting integrated circuits will not be functional.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a block diagram of an EUV system, in accordance with someembodiments.

FIGS. 2A and 2B are illustrations of an EUV system, in accordance withsome embodiments.

FIGS. 3A-3C are graphs of voltage waveforms, in accordance with someembodiments.

FIG. 4 is a graph illustrating droplet coalescence, in accordance withsome embodiments.

FIG. 5A is a graph of a voltage waveform, in accordance with someembodiments.

FIG. 5B is a graph illustrating droplet coalescence in connection withthe voltage waveform of FIG. 5A, in accordance with some embodiments.

FIG. 6A is a graph of a voltage waveform, in accordance with someembodiments.

FIG. 6B is a graph illustrating droplet coalescence in connection withthe voltage waveform of FIG. 6A, in accordance with some embodiments.

FIGS. 7A-7C are graphs of voltage waveforms, in accordance with someembodiments.

FIG. 8 is a flow diagram of a method for operating an EUVphotolithography system, in accordance with some embodiments.

FIG. 9 is a flow diagram of a method for operating an EUVphotolithography system, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of thedisclosure. However, one skilled in the art will understand that thedisclosure may be practiced without these specific details. In otherinstances, well-known structures associated with electronic componentsand fabrication techniques have not been described in detail to avoidunnecessarily obscuring the descriptions of the embodiments of thepresent disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

The use of ordinals such as first, second and third does not necessarilyimply a ranked sense of order, but rather may only distinguish betweenmultiple instances of an act or structure.

Reference throughout this specification to “some embodiments” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least someembodiments. Thus, the appearances of the phrases “in some embodiments”or “in an embodiment” in various places throughout this specificationare not necessarily all referring to the same embodiment. Furthermore,the particular features, structures, or characteristics may be combinedin any suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

Embodiments of the present disclosure provide many benefits to EUVphotolithography systems. Embodiments of the present disclosure generatedroplets in a manner that ensures that the droplets are in a desirablestate before being irradiated with a laser. A droplet generator outputsgroups of droplets. A spread of velocities is imparted to the dropletsof each group so that the droplets coalesce into a single large dropletprior to irradiation by the laser. The droplet generator includes anozzle surrounded by a piezoelectric structure. A voltage waveform isapplied to the piezoelectric structure to cause the piezoelectricstructure to expand or contract around the nozzle. The expansion andcontraction of the piezoelectric structure imparts different velocitiesto the droplets of each group in accordance with the voltage waveform.The voltage waveform includes a combination of a sine wave and a stepfunction. This combination ensures that the droplets of a group have aspread of velocities that will cause coalescence of the droplets into asingle large droplet prior to irradiation by the laser. Because thedroplets coalesce into a single large droplet prior to irradiation, theirradiated large droplet emits a large amount of EUV light. The resultis that photolithography processes are properly performed, leading toincreases in wafer yields and better performance of integrated circuits.

FIG. 1 is a block diagram of an EUV photolithography system 100,according to some embodiments. As will be set forth in more detailbelow, the components of the EUV photolithography system 100 cooperateto generate properly formed droplets and EUV light. As used herein, theterms “EUV light” and “EUV radiation” can be used interchangeably.

The EUV photolithography system 100 includes a droplet generator 102, anEUV generation chamber 104, a droplet receiver 106, a scanner 108, alaser 112, and a collector. The droplet generator 102 outputs dropletsinto the EUV generation chamber 104. The laser 112 irradiates thedroplets with pulses of laser light within the EUV generation chamber104. The irradiated droplets emit EUV light. The EUV light is collectedby a collector 114 and reflected toward the scanner 108. The scanner 108conditions the EUV light and focuses the EUV light onto the target 110.The target 110 may include a semiconductor wafer. After irradiation bythe laser 112, the droplets exit the EUV generation chamber 104 and arereceived by the droplet receiver 106. Further details regarding each ofthese components and processes are provided below.

The droplet generator 102 generates and outputs a stream of droplets.The droplets can include tin, though droplets of other material can beutilized without departing from the scope of the present disclosure. Thedroplets move at a high rate of speed toward the droplet receiver 106.The droplets have an average velocity between 60 m/s to 200 m/s. Thedroplets have an initial diameter between 10 μm and 200 μm. The dropletgenerator can generate different numbers of droplets per second thandescribed above without departing from the scope of the presentdisclosure. The droplet generator 102 can also generate droplets havingdifferent initial velocities and diameters than those described abovewithout departing from the scope of the present disclosure.

More particularly, the droplet generator 102 generates groups ofdroplets. The droplets of each group are initially separate from eachother when leaving the droplet generator 102. As the droplets of a grouptravel through the EUV generation chamber 104, the droplets jointogether into a single large droplet. As will be set forth in moredetail below, the droplet generator 102 imparts different initialvelocities to the droplets of a group. The droplets of a group areoutput one at a time in a sequence. The initial velocity of the dropletsof a group increases with successive droplets. The spread of velocitiesensures that the droplets will join together. In particular, the spreadof velocities is selected to ensure that the droplets of a group joininto a single droplet prior to arriving at a location within the EUVgeneration chamber 104 at which irradiation by the laser 112 occurs.Further details regarding the spread of velocities and how the spread ofvelocities is imparted are set forth below.

In some embodiments, the EUV generator 102 is a laser produced plasma(LPP) EUV generation system. As the droplets travel through the EUVgeneration chamber 104 between the droplet generator 102 and the dropletreceiver 106, the droplets are irradiated by the laser 112. When adroplet is irradiated by the laser 112, the energy from the laser causesthe droplet to form a plasma. The plasmatized droplets generate EUVlight. This EUV light is collected by the collector 114 and passed tothe scanner 108 and then on to the target 110.

In some embodiments, the laser 112 is positioned external to the EUVgeneration chamber 104. During operation, the laser 112 outputs pulsesof laser light into the EUV generation chamber 104. The pulses of laserlight are focused on a point through which the droplets pass on theirway from the droplet generator 102 to the droplet receiver 106. Eachpulse of laser light is received by a droplet. When the droplet receivesthe pulse of laser light, the energy from the laser pulse generates ahigh-energy plasma from the droplet. The high-energy plasma outputs EUVlight.

In some embodiments, the laser 112 irradiates the droplet with twopulses. A first pulse causes the droplet to flatten into a disk likeshape. The second pulse causes the droplet to form a high temperatureplasma. The second pulse is significantly more powerful than the firstpulse. The laser 112 and the droplet generator 102 are calibrated sothat the laser emits pairs of pulses such that the droplet is irradiatedwith a pair of pulses. The laser can irradiate droplets in a mannerother than described above without departing from the scope of thepresent disclosure. For example, the laser 112 may irradiate eachdroplet with a single pulse or with more pulses than two. In someembodiments, there are two separate lasers 112. A first laser 112delivers the flattening pulse. A second laser 112 delivers theplasmatized pulse.

In some embodiments, the light output by the droplets scatters randomlyin many directions. The photolithography system 100 utilizes thecollector 114 to collect the scattered EUV light from the plasma andoutput the EUV light toward the scanner 108.

The scanner 108 includes scanner optics. The scanner optics include aseries of optical conditioning devices to direct the EUV light to thereticle. The scanner optics may include refractive optics such as a lensor a lens system having multiple lenses (zone plates). The scanneroptics may include reflective optics, such as a single mirror or amirror system having multiple mirrors. The scanner optics direct theultraviolet light from the EUV generator 102 to a reticle.

The ultraviolet light reflects off of the reticle back toward furtheroptical features of the scanner optics. In some embodiments, the scanneroptics include a projection optics box. The projection optics box mayhave refractive optics, reflective optics, or combination of refractiveand reflective optics. The projection optics box may include amagnification less than 1, thereby reducing the patterned image includedin the EUV light reflected from the reticle. The projection optics boxdirects the EUV light onto the target 110, for example, a semiconductorwafer.

The EUV light includes a pattern from the reticle. In particular, thereticle includes the pattern to be defined in the target 110. After theEUV light reflects off of the reticle, the EUV light contains thepattern of the reticle. A layer of photoresist typically covers thetarget during extreme ultraviolet photolithography irradiation. Thephotoresist assists in patterning a surface of the semiconductor waferin accordance with the pattern of the reticle.

The droplet generator 102 includes a nozzle 118. The nozzle 118 outputsthe droplets. The droplet generator 102 also includes a piezoelectricstructure 120 coupled to the nozzle 118. The piezoelectric structure 120may surround the nozzle 118 such that the piezoelectric structure 120 iscoaxial with the nozzle 118. If the nozzle 118 is cylindrical, thepiezoelectric structure 120 may also be cylindrical. The piezoelectricstructure 120 is made of a piezoelectric material that expands orcontracts based on applied voltages. In some embodiments, the piezoelectric structure includes a piezoelectric material such as leadzirconate titanate (PZT) or another suitable piezoelectric material.

The piezoelectric structure 120 contracts and expands based on a voltagethat is applied to the piezoelectric structure 120. For example,positive voltages may cause expansion of the piezoelectric structure120. Negative voltages may cause contraction of the piezoelectricstructure 120. Depending on the type of the piezoelectric structure,negative voltages may cause expansion of the piezoelectric structure 120and positive voltages may cause contraction of the piezoelectricstructure 120. Contraction of the piezoelectric structure may result inincreased pressure exerted on the nozzle 118. Expansion of thepiezoelectric structure 120 may result in decreased pressure exerted onthe nozzle 118. Accordingly, the application of voltages to thepiezoelectric structure 120 can influence the nozzle 118.

The piezoelectric structure 120 is coupled to a control system 116. Thecontrol system 116 controls the application of voltages to thepiezoelectric structure 120. In particular, the control system 116 cancontrol a voltage waveform applied to the piezoelectric structure 120.In some embodiments, the power source is coupled to the piezoelectricstructure 120 and to the control system 116. The control system 116controls the power source to apply the voltage waveform to thepiezoelectric structure 120.

The control system 116 can include one or more processors, one or morememories, and communication resources. The control system 116 cancontrol the various components of the EUV photolithography system 100.The control system 116 can control the components in an automated manneror by executing commands input by an operator. The control system canincludes components and resources in disparate locations or in a singlelocation.

When the piezoelectric structure 120 expands, droplet speed decreases.When the piezoelectric structure contracts droplet speed increases. Thechanges in droplet speed may be likened to the velocity of a fluidflowing through a tube. The flow rate of the fluid is constantthroughout the tube, but the velocity of the fluid changes with changesin diameter of the tube. When the diameter of the tube increases, thevelocity of the fluid decreases. When the diameter of the tubedecreases, the velocity of the fluid increases. In a similar manner, theincreases and decreases in pressure exerted by the piezoelectricstructure 120 on the nozzle 118 cause decreases and increases invelocity of the droplets through the nozzle 118. This is the result ofsonic pressure pulses within the nozzle 118 caused by expansion andcontraction of the piezoelectric structure 120.

The control system 116 applies a voltage waveform to the piezoelectricstructure 120 selected to induce a spread of velocities in the dropletsof a group. The waveform makes one cycle for each group of droplets. Atthe beginning of the waveform, the voltage is high. The initialhigh-voltage causes expansion of the piezoelectric structure 120,resulting in the initial droplets having a relatively slow velocity. Thevoltage waveform gradually decreases from a high voltage to alow-voltage. As the waveform decreases from the high-voltage to thelow-voltage, each successive droplet receives a higher velocity until afinal droplet receives the highest velocity. A droplet that passesthrough the nozzle when the waveform is at 0 V receives an averagevelocity v_(a). The velocity of the first droplet of the group isv_(a)+Δv. The velocity of the final droplet of the group is V minus Δv.All droplets between the first droplet and the final droplet have avoltage somewhere between v_(a)+Δv and v_(a)−Δv. The average velocityv_(a) may also be considered as the velocity of the center of mass ofthe group of droplets.

The spread of velocities is imparted to the droplets of the group by thewaveform applied to the piezoelectric structure 120 that causes all ofthe droplets to combine into a single droplet as the droplets passthrough the EUV generation chamber 104. Because later droplets arefaster than earlier droplets in a group, the later droplets catch upwith and coalesce with the earlier droplets until all droplets havejoined together into a single droplet and traveling with the averagevelocity v_(a). Δv and v_(a) are selected to ensure that the droplets ofa group combine into a single droplet prior to crossing the point in theEUV generation chamber 104 at which the laser pulses are received fromthe laser 112.

In some embodiments, the voltage waveform applied to the piezoelectricstructure is a combination of a first voltage waveform and a secondvoltage waveform. The first voltage waveform is a step function. Thesecond voltage waveform is half of a sine wave. The waveform begins at apositive voltage amplitude and remains at the positive voltage amplitudefor a brief period of time and then begins to decline in accordance witha sine wave declining from the positive voltage amplitude to a negativevoltage amplitude

FIGS. 2A-2B are illustrations of a photolithography system 200,according to an embodiment. The photolithography system 200 is anextreme ultraviolet photolithography system that generates extremeultraviolet radiation by laser plasma interaction. The plasma can begenerated in a substantially similar manner as described in relation toFIG. 1 . FIG. 2A illustrates the photolithography system 200 without theextreme ultraviolet radiation. FIG. 2B illustrates the photolithographysystem 200 with the extreme ultraviolet radiation.

With reference to FIG. 2A, the photolithography system 200 includes aplasma generation chamber 104, a laser 112, a scanner 103, a collector114, a droplet generator 102, and a droplet receiver 106. The EUVgeneration chamber 104 is defined by the collector 114 and an enclosure124 coupled to the collector 114. The EUV photolithography system 200includes a droplet generator 102 and a droplet receiver 106. Thecomponents of the photolithography system 200 cooperate together togenerate extreme ultraviolet radiation and to perform photolithographyprocesses with the extreme ultraviolet radiation.

The droplet generator 102 generates and outputs groups 126 of droplets128. The droplets can include, tin, though droplets of other materialcan be utilized without departing from the scope of the presentdisclosure. The droplets 128 move at a high rate of speed toward thedroplet receiver 106.

The droplet generator 102 periodically emits a group 126 of droplets128. The view of FIG. 2A illustrates three groups 126 of droplets 128that have not yet coalesced into a single droplet 126. The view of FIG.2A illustrates, at a laser irradiation point location within the EUVgeneration chamber 104, a large droplet 128 that has coalesced from agroup 126 of droplets 128. The view of FIG. 2A illustrates three largedroplets 128 that have coalesced and have passed beyond the laserirradiation location. In the view of FIG. 2A, each group 126 initiallyincludes four droplets 128. However, in practice, each group 126 mayinclude many more droplets than four. In some embodiments, each group126 includes 50 or more droplets 128.

The piezoelectric structure 120 is coupled to the nozzle 118. Thepiezoelectric structure 120 surrounds a portion of the nozzle 118. Thepiezoelectric structure 120 is coaxial with the nozzle 118. As describedpreviously, the piezoelectric structure 120 expands when a positivevoltage is applied to the piezoelectric structure. The piezoelectricstructure 120 contracts when a negative voltage is applied to thepiezoelectric structure 120. The expansion and contraction of thepiezoelectric structure 120 helps to impart a spread of velocities tothe droplets 128 in each group 126.

A voltage source 122 is coupled to the piezoelectric structure 120. Thevoltage source 122 applies a voltage to the piezoelectric structure 120.In particular, the voltage source 122 applies a repeating voltagewaveform to the piezoelectric structure 120. The nozzle 118 outputs agroup 126 of droplets 128 during each cycle of the voltage waveform. Thevoltage waveform is selected to ensure that droplets at the rear of thegroup 126 have higher velocities than droplets at the front of the group126.

In some embodiments, the voltage waveform is a combination of a stepfunction and half of a sine wave. The voltage is initially at a highvoltage and remains at the high voltage for a brief period at thebeginning of the waveform. The voltage then begins to decline from thehigh voltage to a low voltage in the form of a sine wave. Once thevoltage waveform arrives at the low-voltage, the voltage waveformremains at the low-voltage for a brief period of time until the end ofthe voltage waveform. At the end of the voltage waveform, the voltagechanges from the low-voltage to the high voltage after the manner of astep function. Further details regarding the voltage waveform are givenin relation to FIG. 3 . Other waveforms can be utilized withoutdeparting from the scope of the present disclosure.

As can be seen in FIG. 2A, the droplets 128 of the group 126 closest tothe nozzle 118 are spaced farther apart than are the droplets 128 of thenext closest group 126. This is because the selected spread ofvelocities ensures that droplets 128 at the back of a group catch upwith and join together with droplets at the front of a group 126 as thedroplets travel toward the laser irradiation point. The group 126 thirdclosest to the nozzle 118 has begun coalescing into a larger droplet128. The large droplet 128 at the laser irradiation point corresponds toa group 126 of droplets 128 that have entirely coalesced into a singledroplet 128.

After passing through the laser irradiation point, the coalesceddroplets 128 are received by the droplet receiver 106. The dropletreceiver 106 may include a droplet reservoir 127. The droplets 128travel into the droplet receiver 106, impact a back wall of the dropletreceiver 106, and drop into the droplet reservoir 127. Otherconfigurations for a droplet receiver 106 can be utilized withoutdeparting from the scope of the present disclosure

The laser 112 is positioned behind the collector 114. During operation,the laser 112 outputs pulses of laser light 130. The pulses of laserlight 130 are focused on a point through which the droplets pass ontheir way from the droplet generator 102 to the droplet receiver 106.Each pulse of laser light 130 is received by a fully coalesced droplet128 at the laser irradiation point. When a droplet 128 receives thepulse of laser light 130, the energy from the laser pulse generates ahigh-energy plasma from the droplet 128. The high-energy plasma outputsextreme ultraviolet radiation.

In some embodiments, the laser 112 is a carbon dioxide (CO₂) laser. TheCO₂ laser emits radiation or laser light 130 with a wavelength centeredaround 9.4 μm or 10.6 μm. The laser 112 can include lasers other thancarbon dioxide lasers and can output radiation with other wavelengthsthan those described above without departing from the scope of thepresent disclosure.

In some embodiments, the laser 112 irradiates each droplet 128 with twopulses. A first pulse causes the droplet 128 to flatten into a disk likeshape. The second pulse causes the droplet 128 to form a hightemperature plasma. The second pulse is significantly more powerful thanthe first pulse. The laser 112 and the droplet generator 102 arecalibrated so that the laser 112 emits pairs of pulses such that eachdroplet 128 is irradiated with a pair of pulses. The laser 112 canirradiate droplets 128 in a manner other than described above withoutdeparting from the scope of the present disclosure. For example, thelaser 112 may irradiate each droplet 128 with a single pulse or withmore pulses than two. Moreover, the primary laser here can not onlycause droplet into disk-like shape but also can be mist or vapor state.

FIG. 2B illustrates EUV light 132 being emitted from the droplet 128receiving the laser light pulse 130. When the droplets 128 are convertedto a plasma, the droplets 128 output EUV light 132. In an example inwhich the droplets 128 are tin, the droplets 128 output EUV light 132with a wavelength centered between 10 nm and 15 nm. More particularly,in some embodiments, the tin plasma emits EUV light with a centralwavelength of 13.5 nm. Materials other than tin can be used for thedroplets 128 without departing from the scope of the present disclosure.Such other materials may generate extreme ultraviolet radiation withwavelengths other than those described above without departing from thescope of the present disclosure.

In some embodiments, the EUV light 132 output by the droplets 128scatters in many directions. The photolithography system 100 utilizesthe collector 114 to collect the scattered EUV light 132 from the plasmaand output the EUV light 132 toward a photolithography target.

In some embodiments, the collector 114 is a parabolic or ellipticalmirror. The scattered EUV light 132 is collected and reflected by theparabolic or elliptical mirror with a trajectory toward a scanner (notshown in FIGS. 2A and 2B). The scanner utilizes a series of opticalconditioning devices such as mirrors and lenses to direct the extremeultraviolet radiation to the photolithography mask. The EUV light 132reflects off of the mask onto a photolithography target. The EUV light132 reflected from the mask patterns a photoresist or other material ona semiconductor wafer. For purposes of the present disclosure,particularities of the mask and the various configurations of opticalequipment in the scanner are not shown.

In some embodiments, the collector 114 includes a central aperture 129.The pulses of laser light 130 pass from the laser 112 through thecentral aperture 129 toward the stream of droplets 128. This enables thecollector 114 to be positioned between the laser 112 and the scanner.

FIG. 3A illustrates a graph of a step function voltage waveform 300,according to one embodiment. The x-axis corresponds to time. The y-axiscorresponds to voltage. At time to, the step function voltage waveform300 is initially at a negative voltage amplitude V_(n). At time t₁, thestep function voltage waveform 300 changes, in a discontinuous manner,from the negative voltage amplitude V_(n) to a positive voltageamplitude V_(p). In some embodiments, the negative voltage amplitudeV_(n) is equal to the positive voltage amplitude V_(p) multiplied by −1.The step function voltage waveform 300 continues at the positive voltageamplitude V_(p) until time t₂. The relevance of the step functionvoltage waveform 300 to the EUV photolithography system will be madeclearer with reference to FIG. 3C.

FIG. 3B illustrates a graph of a half sine wave voltage waveform 302,according to one embodiment. At time to, the half sine wave voltagewaveform 302 is at a positive voltage amplitude V_(p). Between times t₀and time t₁, the half sine wave voltage waveform 302 decreases from thepositive voltage amplitude V_(p) to a negative voltage amplitude V_(n).The relevance of the half sine wave voltage waveform 302 to the EUVphotolithography system will be made clearer with reference to FIG. 3C.

FIG. 3C is a graph of a voltage waveform 304 applied to thepiezoelectric structure 120 of FIG. 1 or of FIG. 2A or 2B, according toone embodiment. The x-axis corresponds to time. The y-axis correspondsto voltage. The voltage waveform 304 is a combination of the squarefunction voltage waveform 300 of FIG. 3A and the half sine wave voltagewaveform 302 of FIG. 3B. The voltage waveform 304 is a repeating voltagewaveform with a period T.

Times t₀ to t₄ illustrate a single period T of the voltage waveform 304.At time to, the voltage waveform 304 is initially at a positive voltageamplitude V_(p). The voltage waveform 304 remains at the positivevoltage amplitude V_(p) until time t₁. The portion of the voltagewaveform 304 between times t₀ and t₁ corresponds to the portion of thesquare wave voltage waveform 300 of FIG. 3A between times t₁ and t₂. Attime t₁, the voltage waveform 304 begins to decrease from the positivevoltage amplitude V_(p) toward a negative voltage amplitude V_(n). Attime t₂, the voltage waveform 304 is a 0 V. At time t₃, the voltagewaveform 304 reaches the negative voltage amplitude V_(n). The portionof the voltage waveform 304 between times t₁ and t₃ corresponds to thehalf sine wave voltage waveform 302 of FIG. 3B. Between times t₃ and t₄,the voltage waveform 304 remains constant at the negative voltageamplitude V_(n). The portion of the waveform 304 between times t₃ and t₄corresponds to the portion of the square wave voltage waveform 300 ofFIG. 3A between times t₀ and t₁. At time t₄, the voltage waveform 304jumps to the positive voltage amplitude V_(p) in the manner of a stepfunction. Accordingly, the voltage waveform 304 corresponds to acombination of the square wave voltage waveform 300 of FIG. 3A and thehalf sine wave voltage waveform 302 of FIG. 3B.

Each cycle of the voltage waveform 304 has a period T. In one example,the period T is between 10 μs and 30 μs. Each cycle of the voltagewaveform 304 is applied to the piezoelectric structure 120 whileoutputting a group 126 of droplets 128 from the nozzle 118. The droplets128 that are output during times t₀ and t₁ will have a velocity equal toan average velocity v_(a)−Δv. The droplets 128 output between times t₁and t₃ will have increasing velocities. In other words, between times t₁and t₃ each droplet 128 will have a higher velocity than the droplet 128immediately preceding. A droplet 128 output at time t₂ will have theaverage velocity v_(a). A droplet 128 output at time t₃ will have avelocity equal to the average velocity v_(a)+Δv. Droplets output betweentimes t₃ and t₄ will all have the velocity v_(a) plus Δv.

The positive voltage amplitude V_(p) and the negative voltage amplitudeV_(n) are selected to provide a value of Δv that causes all of thedroplets 128 to coalesce into a single droplet before arriving at thelaser irradiation point. In some embodiments, the positive voltageamplitude V_(p) is between 3 V and 12 V. The negative voltage amplitudeV_(n) is between −3 V and −12 V. A voltage between these amplitudes maybe sufficient to ensure coalescence of the droplets 128 of the group 126prior to arriving at the laser irradiation point in an example in whichthe laser irradiation point is about 0.5 m from the nozzle 118 and theaverage velocity v_(a) is 80 m/s. Other values of V_(p) and V_(n) can beutilized without departing from the scope of the present disclosure.Other values of the average velocity v_(a) and the distance between thenozzle 118 and the laser irradiation point can be utilized withoutdeparting from the scope of the present disclosure. In particular, thevalues of V_(p) and V_(n) can be selected based on the particularconfiguration of the nozzle 118, the average velocity v_(a), the initialdistance between the first and last droplets 128 of a group 126, and thedistance between the nozzle 118 and the laser irradiation point. In someembodiments, the first droplet 128 and the last droplet 128 of a group126 are initially separated by a distance between 0.5 mm and 3 mm,though other initial separation distances can be utilized withoutdeparting from the scope of the present disclosure.

FIG. 4 is a graph 400 illustrating the coalescence of a group 126 ofdroplets 128 of an EUV photolithography system, according to someembodiments. The x-axis corresponds to the distance between a droplet128 and the center of mass of the group 126 of droplets 128. The y-axiscorresponds to the time of flight of the droplets. FIG. 4 alsoillustrates the droplet generator 102 including the nozzle 118 thatoutputs the droplets 128, and the piezoelectric structure 120 thatreceives the voltage waveform.

At time to, all the droplets 128 of the group 126 have been output fromthe nozzle 118. The value λ corresponds to the total distance betweenthe first droplet 128 and the last droplet 128. Accordingly, the lastdroplet 128 is at a value of negative λ/2 from the center of mass of thegroup 126. The first droplet 128 is at a value of λ/2 from the center ofmass of the group 126. As described previously, the droplets 128 willhave different velocities. In general, later droplets 128 will havehigher velocities than earlier droplets 128 of the group 126. Thedroplet 128 furthest to the right at time to corresponds to the first orearliest droplet 128. The droplet 128 furthest to the left at time tocorresponds to the last or latest droplets 128 of the group 126.

At time t₁ some of the droplets 128 have coalesced at the center ofmass. This corresponds to the center of mass catching up to some of theearlier droplets 128 and some of the later droplets 128 catching up tothe center of mass. At time t₂, the center of mass has caught up to moreof the earlier droplets 128 and more of the later droplets 128 havecaught up to the center mass. However, at time t₂, the center mass isnot caught up to the earliest droplets 128 and the latest droplets 128have not caught up to the center mass. At time t₃ the center mass hasfinally caught up to the earliest fire droplets 128 and the latest firedroplets 128 have finally caught up to the center mass of the group 126.Accordingly time t₃ corresponds to the total coalescence time t_(c) ofthe group 126 of droplets 128 the total coalescence time is the time atwhich all of the droplets 128 of a group 126 have coalesced into asingle droplet 128 having the mass of the whole group 126.

As set forth previously, it is desirable for total coalescence to occurbefore the center of mass reaches the laser irradiation point. This willdepend on the average velocity of the droplets 128 (or the velocity ofthe center mass), the value of Δv, the initial distance λ between thefirst and last droplets 128, and the distance between the nozzle 118 andthe laser irradiation point. The coalescence length l_(c) is given bythe following formulas:

l_(c) = t_(l)^(*)v_(a) = (λ/2^(*)v_(a))/Δv.Δv is given by the following relationship:

Δv = V_(p)^(*)TF,where TF is the transfer function associated with the voltage applied tothe piezoelectric structure 120 and the change in velocity imparted tothe droplets within the nozzle 118 based on the piezoelectric structure120.

FIG. 5A illustrates a sine wave voltage waveform 500. This is onevoltage waveform that could be applied to the piezoelectric structure120. However, as will be set forth with relation to FIG. 5B, the sinewave voltage waveform 500 has some drawbacks compared to the voltagewaveform 304 of FIG. 3C. At time to, the voltage is 0 V. Accordingly,droplets 128 output from the nozzle 118 at or near this time will have avelocity equal to or nearly equal to the average velocity v_(a) or thevelocity of the center mass. At time t₁ the voltage is at V_(p).Accordingly, droplets 128 output near time t₁ will have a much lowervelocity than v_(a). At time t3 the waveform reaches the negativevoltage amplitude V_(a). Accordingly, droplets 128 output near time t₃will have a much higher velocity than v_(a). At time t₄, the waveformhas returned to 0 V. Accordingly, droplets 128 output near time t₄ willhave velocities equal to or nearly equal to the average velocity v_(a).

The droplets output between times t₁ and t₃ may have no problemcoalescing with the center of mass. However, the droplets output neartimes t₀ and t₄ may not coalesce with the center of mass at all becausethey are velocities or near the average velocity v_(a). This isillustrated with reference to FIG. 5B. In one example, the periodbetween times t₀ and t₄ is about 20 μs. Accordingly, time t₁ is about 5μs. Time t₂ is about 10 μs. Time t₃ is about 15 μs. Time t₄ is about 20μs. Other time periods can be utilized without departing from the scopeof the present disclosure.

FIG. 5B is a graph 502 illustrating the coalescence of droplets 128 ingroup 126 output from the nozzle 118 while the waveform 500 is appliedto the piezoelectric structure 120. The x-axis corresponds to thedistance of droplets from the center of mass in millimeters. The y-axiscorresponds to the distance from the nozzle 118. The initial totaldistance between the first droplet 128 and the last droplet 128 of thegroup 126 is λ. In one example, λ is about 1.5 mm, though other valuescan be utilized without departing from the scope of the presentdisclosure. Droplets 128 that are initially within about λ/3 from thecenter of mass are able to coalesce within a relatively short distancefrom the nozzle, for example by about 150 mm away from the nozzle 118.These droplets correspond to the droplets output between times t₁ andt₃. However, the droplets 128 that are initially nearly λ/2 away fromthe center of mass do not quickly coalesced with the center mass. Thisis because these droplets 128 were output near t₀ or t₄ and havevelocities nearly equal to the velocity of the center of mass.Accordingly, the earliest and latest droplets 128 will not coalescebefore the laser irradiation point.

FIG. 6A is a graph of a voltage waveform 600, according to oneembodiment. The voltage waveform 600 is substantially similar to thevoltage waveform 304 of FIG. 3C. In particular, the voltage waveform 600is a combination of a step function and a half sine wave. Benefits ofthis waveform are shown in relation to FIG. 6B. In some embodiments, theperiod T of the waveform 600 is 20 μs. The contrast between the waveform500 and the waveform 600 can be seen most particularly between times t₀and t₁ and between times t₃ and t₄. In the waveform 500, the voltage attimes t₀, t₂, and t₄ is 0 V, corresponding to an imparted velocitynearly equal to the velocity of the center mass. To the contrary, in thewaveform 600 the voltage at time to is the positive voltage amplitudeV_(p). The voltage at time t₄ is the negative voltage amplitude V_(n).Accordingly, droplets 128 that leave the nozzle at time to have animparted velocity that is significantly slower than the velocity of thecenter of mass. Droplets 128 that leave the nozzle at time t₄ have animparted velocity that is significantly faster than the velocity of thecenter of mass. The result is much quicker coalescence of the entiregroup 126 of droplets 128.

FIG. 6B is a graph illustrating coalescence of a group 126 of droplets,according to one embodiment. The group 126 of droplets 128 is outputfrom the nozzle 118 while the waveform 600 (or 304) is applied to thepiezoelectric structure surrounding the nozzle 118. The y-axiscorresponds to distance from the nozzle 118 in millimeters. The x-axiscorresponds to distance of the droplets 128 from center mass of thegroup 126. The first droplet 128 is initially λ/2 away from the centerof mass of the group 126. The final droplet 128 is initially −λ/2 awayfrom the center mass of the group 126. In one example, λ is about 1.5mm. In some embodiments, the result is a coalescence length l_(c) lessthan 200 mm. In some embodiments, about 150 mm. The graph 602 alsoillustrates a length l₂ corresponding to the coalescence length for thewaveform 500, although in the waveform 500 some of the droplets may notcoalesce at all.

FIG. 7A-7C illustrate voltage waveforms that can be applied to thepiezoelectric structure 120 of an EUV photolithography system such asthe EUV photolithography systems 100 and 200 of FIGS. 1 and 2A-2B,according to some embodiments. FIG. 7A illustrates a waveform 700.Between times t₀ and t₁, the waveform 700 has a constant value equal tothe positive voltage amplitude. Between times t₁ and t₂, the waveform700 decreases at a linear rate from V_(p) to the negative voltageamplitude V_(n). Between times t₂ and t₃ the waveform 700 has a constantvalue of V_(n). The period of the waveform 700 corresponds to the timebetween t₀-t₃. At time t₃, the waveform 700 jumps from V_(n) to V_(p) inthe manner of a step function. The waveform 700 can correspond to thecombination of a step function and a linearly decreasing function. Thewaveform 700 may result in coalescence of a group 128 of droplets priorto reaching the laser irradiation point.

FIG. 7B illustrates a waveform 702. Between times t₀ and t₁, thewaveform 702 decreases at a linear rate from V_(p) to V_(n). The periodof the waveform 700 corresponds to the time between t₀-t₁. At time t₁,the waveform 702 jumps from V_(n) to V_(p) in the manner of a stepfunction. The waveform 702 may result in coalescence of a group 128 ofdroplets prior to reaching the laser irradiation point.

FIG. 7C illustrates a waveform 704. Between times t₀ and t₁, thewaveform 704 decreases from V_(p) to V_(n) in the form of a halfsinewave. The period of the waveform 704 corresponds to the time betweent₀-t₁. At time t₁, the waveform 704 jumps from V_(n) to V_(p) in themanner of a step function. The waveform 704 may result in coalescence ofa group 128 of droplets prior to reaching the laser irradiation point.

FIG. 8 is a flow diagram of a method 800 for operating an extremeultraviolet photolithography system, according to some embodiments. At802, the method 800 includes outputting droplets from a dropletgenerator into an extreme ultraviolet radiation generation chamber. Oneexample of droplets is the droplets 128 of FIG. 2A. One example of adroplet generator is the droplet generator 102 of FIG. 2A. One exampleof an extreme ultraviolet generation chamber is the extreme ultravioletradiation generation chamber 104 of FIG. 2A. At 804, the method 800includes applying, to a piezoelectric structure coupled to a nozzle ofthe droplet generator, a voltage waveform including a combination of ahalf sine wave and a step function. One example of a voltage waveform isthe voltage waveform 304 of FIG. 3C. One example of a piezoelectricstructure is the piezoelectric structure 120 of FIG. 2A. One example ofnozzle is the nozzle 118 of FIG. 2A. At 806, the method 800 includesgenerating extreme ultraviolet radiation from the droplets in theextreme ultraviolet radiation generation chamber.

FIG. 9 is a flow diagram of a method 900 for operating an extremeultraviolet photolithography system, according to some embodiments. At902, the method 900 includes outputting a group of droplets from anozzle of a droplet generator into an extreme ultraviolet radiationgeneration chamber. One example of a group of droplets is the group 126of droplets 128 of FIG. 2A. One example of nozzle is the nozzle 118 ofFIG. 2A. One example of an extreme ultraviolet generation chamber is theextreme ultraviolet radiation generation chamber 104 of FIG. 2A. At 904,the method 900 includes causing the group of droplets to merge into asingle droplet within the extreme ultraviolet generation chamber byimparting a spread of velocities to the droplets of each group byapplying, to a piezoelectric structure coupled to the nozzle, a voltagewaveform with a cycle beginning at a positive voltage amplitude andending at a negative voltage amplitude. One example of a voltagewaveform is the voltage waveform 304 of FIG. 3C. One example of apiezoelectric structure is the piezoelectric structure 120 of FIG. 2A.At 906, the method 900 includes generating extreme ultraviolet radiationby irradiating the single droplet with a laser within the extremeultraviolet radiation generation chamber. One example of a laser is thelaser 112 of FIG. 2A. At 908, the method 900 includes performing aphotolithography process on a wafer with the extreme ultravioletradiation.

In one embodiment, a method includes outputting droplets from a dropletgenerator into an extreme ultraviolet radiation generation chamber. Themethod includes applying, to a piezoelectric structure coupled to anozzle of the droplet generator, a voltage waveform including acombination of a half sine wave and a step function. The method includesgenerating extreme ultraviolet radiation from the droplets in theextreme ultraviolet radiation generation chamber.

In one embodiment, a system includes an extreme ultraviolet generationchamber. The system includes a droplet generator configured to outputdroplets into the extreme ultraviolet generation chamber and including anozzle and a piezoelectric structure coupled to the nozzle. The systemincludes a laser configured to irradiate the droplets in the extremeultraviolet generation chamber. The system includes a control systemconfigured to apply to the piezoelectric structure a voltage waveformincluding a combination of a step function and a half sine wave.

In one embodiment, a method includes outputting a group of droplets froma nozzle of a droplet generator into an extreme ultraviolet radiationgeneration chamber and causing the group of droplets to merge into asingle droplet within the extreme ultraviolet radiation generationchamber by imparting a spread of velocities to the droplets of eachgroup by applying, to a piezoelectric structure coupled to the nozzle, avoltage waveform with a cycle beginning at a positive voltage amplitudeand ending at a negative voltage amplitude. The method includesgenerating extreme ultraviolet radiation by irradiating the singledroplet with a laser within the extreme ultraviolet radiation generationchamber and performing a photolithography process on a wafer with theextreme ultraviolet radiation.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method, comprising: outputting droplets from adroplet generator into an extreme ultraviolet radiation generationchamber; applying, to a piezoelectric structure coupled to a nozzle ofthe droplet generator, a voltage waveform including a combination of ahalf sine wave and a step function; and generating extreme ultravioletradiation from the droplets in the extreme ultraviolet radiationgeneration chamber.
 2. The method of claim 1, wherein outputting thedroplets includes outputting a group of droplets during each cycle ofthe waveform.
 3. The method of claim 2, further comprising imparting aspread of velocities to the droplets of each group by applying thewaveform to the piezoelectric structure.
 4. The method of claim 3,wherein the spread of velocities causes the droplets of each group tocoalesce into a single droplet.
 5. The method of claim 4, whereinimparting the spread of velocities causes a first droplet in each groupto have a lower velocity than a final droplet in each group.
 6. Themethod of claim 4, further comprising generating the extreme ultravioletradiation by irradiating the single droplet of each group with a laser.7. A system, comprising: an extreme ultraviolet radiation generationchamber; a droplet generator configured to output droplets into theextreme ultraviolet radiation generation chamber and including: anozzle; and a piezoelectric structure coupled to the nozzle; a laserconfigured to irradiate the droplets in the extreme ultravioletradiation generation chamber; and a control system configured to applyto the piezoelectric structure a voltage waveform including acombination of a step function and a half sine wave.
 8. The system ofclaim 7, wherein the droplets include liquid tin.
 9. The system of claim7, wherein the control system is configured to apply the voltagewaveform while the droplet generator is generating droplets.
 10. Thesystem of claim 9, wherein the droplet generator is configured to outputa group of droplets during each cycle of the voltage waveform.
 11. Thesystem of claim 10, wherein the voltage waveform causes the droplets ofeach group to coalesce into a single droplet prior to irradiation withthe laser.
 12. The system of claim 9, wherein the voltage waveformcauses the droplets of each group to coalesce less than 200 mm from thenozzle.
 13. The system of claim 7, wherein the piezoelectric structuresurrounds the nozzle.
 14. The system of claim 13, wherein the voltagewaveform causes the piezoelectric structure to generate a sonic pulsewithin the nozzle.
 15. A method, comprising: outputting a group ofdroplets from a nozzle of a droplet generator into an extremeultraviolet radiation generation chamber; causing the group of dropletsto merge into a single droplet within the extreme ultraviolet generationchamber by imparting a spread of velocities to the droplets of eachgroup by applying, to a piezoelectric structure coupled to the nozzle, avoltage waveform with a cycle beginning at a positive voltage amplitudeand ending at a negative voltage amplitude; generating extremeultraviolet radiation by irradiating the single droplet with a laserwithin the extreme ultraviolet radiation generation chamber; andperforming a photolithography process on a wafer with the extremeultraviolet radiation.
 16. The method of claim 15, wherein outputtingthe group of droplets includes outputting the droplets consecutively.17. The method of claim 16, wherein the spread of velocities includescausing each droplet other than a first droplet to have a highervelocity than an immediately preceding droplet.
 18. The method of claim17, wherein the waveform includes decreasing the voltage from a positiveamplitude with the half sine wave to a low amplitude and then increasingfrom the low amplitude to the high amplitude with the step function. 19.The method of claim 18, further comprising generating a sonic pulsewithin the nozzle by applying the voltage waveform.
 20. The method ofclaim 15, wherein the group of droplets includes more than 50 droplets.